It sometimes occurs in digital telecommunications systems that customer-premises communications equipment is timed independently of the network communications equipment (e.g., the public switched telephony network) that interconnects the customer-premises equipment. A particularly significant example thereof is the code-division multiplexed-access (CDMA) radio-telephone system, which is an important type of digital cellular mobile-telephone system. In the CDMA system, nodes that contain radios, i.e., the mobile radio-telephones and cell-site base stations (cells for short), are synchronized to clock signals received by the cells from a global-positioning system (GPS) satellite, whereas the radio-telephone switching systems which interconnect the base stations with each other and with the public telephone network by means of digital communications are synchronized to clock signals which may also be received from the GPS satellite but are distributed by the telephone network.
For purposes of this discussion, two series of events, signals, or operations are considered to be synchronized with each other, or synchronous, if (a) they either occur at the same nominal frequency or one occurs at a frequency that is an integral multiple of the frequency of the other, and (b) they occur in a fixed phase relationship with one another. Operations that are not synchronous are considered to be asynchronous for purposes of this discussion.
The independent timing of the operation of different units of a communication system destroys the assumption that the units provide call traffic to each other at a predetermined steady and unvarying frequency at steady and unvarying points in time i.e. a fixed phase. Rather, independent timing results in the units providing call traffic to each other at a rate and at points in time that fluctuate about a fixed frequency and phase. This asynchrony must be compensated for somehow.
Independent timing is but one cause of this asynchrony. Another cause that may be present in communication systems, such as the CDMA radio-telephone system mentioned above, is the lack of a predetermined and fixed transmission delay between the communicating units. Assuming that both the originating and the destination units are timed either by a common clock or by different clocks that are synchronized with each other, if the transmission delay between the units is fixed and pre-determinable, it can be compensated for in the communication system design such as to allow the units to operate synchronously with each other. But if the delay cannot be predetermined but is variable and fluctuates, the net effect is the same as if the units were independently timed. The fluctuation in the delay may be a result of, for example, occasional changes in the transmission paths that are followed by communications moving between communicating units, or variances in the communication traffic load that flows between--and that must be handled by--the communicating units. This asynchrony must likewise be compensated for.
A partial though inadequate solution to the problems caused by independent timing is to conduct communications between the communicating units in analog instead of digital form. Analog communications can be received asynchronously with their transmission. And while the asynchrony may introduce errors or "glitches" into the communications, the problem is often tolerable for voice-only communications. Thus, in the CDMA radio-telephone system, the radio-telephone switching systems may also be synchronized to the GPS satellite clock signals and hence operate synchronously with the radio-telephones and base stations, if the switching systems are interfaced to the telephone network via analog voice-only communications. Of course, such an arrangement suffers all of the disadvantages that are associated with analog communications, such as low quality and capacity and susceptibility to interference, plus the problem of asynchrony-induced glitches that make the arrangement unsuitable for data communications.
Likewise, a partial though inadequate solution to the problems caused by fluctuating transmission delays is to circuit-switch communication traffic, whereby the dependency of transmission delay on communication traffic load is avoided. However, circuit-switching is inefficient or undesirable for other reasons in many applications. Furthermore, circuit switching does not eliminate fluctuation of transmission delay that is caused by changes in the transmission path, such as will typically arise during CDMA call "soft handoff".